The Oldman Dam, just north of Pincher Creek, Alberta, was the site of
controversy during its' construction in the early 1990s. A group of Peigan
First Nations resisted the dam, claiming it would flood sacred Blackfoot
territory. The protests reached a climax when a group calling themselves the
Peigan Lonefighters fired shots at dam construction workers.
(Photo: First Light)
Although labelled a green energy source, hydro-electric production can have negative effects on the environment
More than half of the world’s large rivers have been dammed, regulating and flooding
approximately 400,000 square kilometres of land worldwide. These diversions have an effect
on diverse ecosystems and habitats around the globe, replacing them with uniform structures
and reservoirs and ultimately changing the way otherwise balanced, stable ecosystems function.
Go with the flow
The life of a river is closely tied to its stream flow, which constantly fluctuates. Damming
a river and altering its flow pattern generates a number of physical and biological impacts.
The disruption of a river’s flow obstructs its’ natural current and affects
the water’s habitat.
One of the largest impacts a lack of current has on a river is the sediment flow, which
is normally carried down the river by the current. When trapped by a dam, the sediment is
held in the reservoir and settles to the bottom while clear water containing very little
sediment is released down the river.
Over time, the easily erodable material from the riverbed is carried away with no sediment
being deposited to replace it. This leaves a rocky streambed, resulting in a poorer habitat
for aquatic fauna.
When flow regimes and sediment loads are altered, the width and depth of the channel are
also affected. A river’s natural meanders, pooling areas and riffles – light
rapids where water flows across a shallow section – are changed by damming and the
modified transportation and distribution of gravel affects fish’ spawning habitats.
The lack of current also exposes young migrating fish to predators for a longer period of
time in the slower moving water.
The surrounding ecosystem is also affected as new floodplains submerge the vegetation, causing
it to decompose. Sometimes wetlands are created, completely altering the ecosystem.
The most visible and obvious effect of dams is that they fragment rivers and make migration
difficult for fish and other aquatic life. Species, such as salmon and eels that migrate
to spawn, may not make it to their destination or may suffer injury or death while travelling
through turbines or over spillways. Fish that do make it through are often disoriented
and become more susceptible to predators.
Some dams are equipped with fish passage structures, or fish ladders, to attempt to accommodate
the migration of a river’s aquatic life. Questions have been raised as to whether fish
ladders are actually too stressful for an adult fish and that its’ chances for successful
spawning is reduced.
When water is held in the reservoir of a dam, the quality of water is affected in several
ways, the extent of which depending on how long it is held there.
The initial creation of a reservoir as a floodplain submerges the existing vegetation and
soil, causing it to decompose and deplete oxygen from the water supply. Mercury, which exists
in a harmless inorganic form in soil, may be transformed by bacteria into methyl mercury
once the soil is flooded. The toxic methyl mercury is lethal to the fish downstream and can
be absorbed and passed up the food chain.
A river’s aquatic community is also sensitive to the temperature of the water. Storing
water in the reservoir of a dam for extended periods of time creates stratification, or layers
of water that are different temperatures. This can affect the nutrient levels, dissolved
oxygen levels and the productivity of heavy metals in the water.
The community is also often cut off from the operation of the dam because the sophisticated
technology of modern dams requires technical expertise that is often taken over by the government
or corporations. The community, therefore, loses any control over the water they once depended
Millions of people have been displaced to make way for the construction of dams around the
world. Communities that have been uprooted are often dispersed, greatly altering their
livelihood, way of life and social network.